1. Field of the Invention
The present invention relates to a throw-away tip (hereinafter referred to as a tip) for use in drilling work adapted to be attached to a throw-away drilling tool or the like for use in drilling work, and also to a throw-away drilling tool (hereinafter referred to as a drilling tool) attached with such a tip.
2. Description of the Related Art
As a tip of this type, there is proposed in Japanese Patent Application No. Hei 11-45482 one as shown in FIGS. 4 and 5. The tip shown in these Figures has a tip body 1 provided in the form of a substantially pentagonal plate, with one of the pentagonal surfaces formed as a rake face 2 and one side surface formed as a flank face 3. A cutting edge 4 is formed at a side ridge of the rake face 2 where the rake face 2 crosses the flank face 3 (along a crossing ridge between the rake face 2 and the flank face 3). The flank face 3 and another side surface 5 of the tip body 1 are disposed in crossed directions at a corner C at one end 4a side of the cutting edge 4. On the rake face 2 on the corner C, there are formed along the side surface 5 in the order from the corner C, a cylindrical surface 2a having a center line parallel to the side surface 5 and the other pentagonal surface constituting a seat face 6 for the tip body 1, which proceeds toward the seat face 6 while convexly curving as it proceeds towards the side surface 5, and a rising surface 2b continuous to the cylindrical surface 2a and gradually rising as it proceeds apart from the corner C side. Consequently, a convexly curved cutting edge portion 4A is formed at one end 4a side of the cutting edge 4, at the crossing ridge between the cylindrical surface 2a and the flank face 3, which cutting edge portion 4A curves convexly in an arc shape as shown in FIG. 5, as viewed from a direction opposing the flank face 3 along the above center line (as viewed in the direction of an arrow E in FIG. 4). The other end 4b side of the cutting edge 4, as viewed from the same direction, smoothly adjoins the arc formed by the convexly curved cutting edge portion 4A of the cutting edge 4 and extends parallel to the seat face 6 to form a straight cutting edge portion 4B.
A pair of the thus constituted tips, as shown in FIG. 6, are mounted at the distal end of a substantially columnar tool body 7 of a throw-away drilling tool rotatable about an axis O, in symmetrical relation about the axis O to be used in drilling work. In other words, a pair of chip discharge grooves 8 and 8 are provided, in symmetrical relation about the axis O, in the outer periphery of the tool body 7 to extend from the distal to proximal end of the tool body 7 while twisting around the axis O rearwardly in a direction of rotation T of the tool. A tip-mounting seat 9 is formed at the distal end of a wall surface 8a of each chip discharge groove 8 facing in the direction of rotation T of the tool, and the tip is mounted on the respective tip-mounting seat 9, with the rake face 2 facing in the direction of rotation T of the tool, the flank face 3 facing the distal end of the tool, and the side surface 5 facing the inner periphery of the tool. The tip has its seat face 6 seated on the bottom surface of a tip-mounting seat and is detachably fixed in place with a clamp screw 10. In this mounted condition, the one end 4a of the cutting edge 4 is located in the vicinity of the axis O at the distal end of the tool body 7 and the other end 4b is located at the outer periphery of the distal end of the tool, to provide the cutting edge 4 with a point angle that proceeds toward the proximal end of the tool as it proceeds toward the tool outer periphery. It is arranged such that the convexly-curved cutting edge portion 4A convexes in the direction of rotation T of the tool.
With a drilling tool with the above tip mounted like this, because the cutting edge 4 at the one end 4a side near the axis O of the tool body 7 is formed in a curve shape projecting in the direction of rotation T of the tool by the convexly-curved cutting edge portion 4A, a reduction is made in the rotational speed and a cutting-edge strength may be secured in the vicinity of the axis O where a great load acts. Incidentally, when a pair of tips are mounted on the tip-mounting seats 9 and 9 at the distal end of the chip discharge grooves 8 and 8, there is formed a very small clearance between the one ends 4a and 4a of their cutting edges 4 and 4 located near the axis O so as to reduce the thrust load of the load as mentioned above. In addition, a recess 7a is provided rearwardly of the clearance, at the center of the distal end of the tool body 7, which is in communication with the chip discharge grooves 8, 8. The provision of such a clearance between the one ends 4a, 4a of the cutting edges 4, 4 allows a core of a work piece that grows along the axis O during drilling work to be received in the recess 7a and discharged through the chip discharge grooves 8, 8. Furthermore, at the tool inner periphery side of each tip-mounting seat 9, a wall surface 9a is formed that is inclined toward the tool outer periphery as it proceeds toward the proximal end (rear end) of the tool. By such wall surfaces 9a, 9a of both tip-mounting seats 9, 9, on a side rearward of the recess 7a at the distal end of the tool body 7, a wall is formed around the axis O that gradually becomes thicker toward the rear end of the tool, so that strength is secured at the distal end of the tool body 7. The tip is mounted, with its side surface 5 that faces the inner periphery of the tool abutting against the related wall surface 9a. 
With the above structure in which the wall surface 9a of the tip-mounting seat 9 inclinedly proceeds toward the tool outer periphery as it proceeds toward the tool distal end, and the tip body 1 is mounted, with its side surface 5 contacted with the wall surface 9a, the center line of the above cylindrical surface 2a, which is in parallel to the side surface 5, also inclinedly proceeds toward the tool outer periphery as it proceeds toward the tool distal end, as viewed from a direction opposing the rake face 2. Hence, the direction in which the one end 4a side portion of the cutting edge 4 formed at the crossing ridge between the cylindrical surface 2a and the flank face 3 looks to form an arc shape, i.e., the direction indicated by the arrow E in FIG. 4 will, as shown for each tip in FIG. 6, also become a direction in which the tip body 1 of one tip is viewed, with an eye cast from near the one end 4a of the cutting edge 4 of the other tip on the opposite side of the axis O toward the outer periphery of the rear end of the tool, rather than the direction of viewing the tip body 1 of the one tip from the tool distal end along the axis O of the tool body 7.
With this structure, however, in which the center line of the cylindrical surface 2a is inclined toward the tool outer periphery as it proceeds toward the tool rear end, and the one end 4a side portion of the cutting edge 4 forms an arc shape when the tip body 1 is viewed in a direction from near the one end 4a toward the outer periphery of the tool rear end, the cylindrical surface 2a drops with a sharp curvature toward the seat face 6 as it proceeds away from the cutting edge 4. Due to this, of the chips produced by the cutting edge 4, that part produced by the straight cutting edge portion 4B at the other end 4b side of the cutting edge 4 flow out in a direction perpendicular to the cutting edge 4, whereas that part produced by the convexly-curved cutting edge portion 4A at the one end 4a side are guided to flow out in that direction in which the cylindrical surface 2a drops or toward the inner periphery of the tool. The flow in different directions of the chips produced at the one end 4a side and at the other end 4b side of the cutting edge 4 causes the chips to be sheared apart in the width direction thereof, and that part of the chips produced by the convexly-curved cutting edge portion 4A are guided along the cylindrical surface 2a toward the inner periphery of the tool and flows into the recess 7a at the center of the distal end of the tool body 1 to be packed therein.
Furthermore, with the conventional tip, because the cylindrical surface 2a of the rake face 2 is formed to drop toward the seat face 6 with a sharp curvature in a direction perpendicular to the cutting edge 4, the included angle of the convexly-curved cutting edge portion 4A at the one end 4a side of the cutting edge 4, in section perpendicular to the cutting edge 4, becomes small, resulting in a reduced cutting-edge strength. Especially because the one end 4a side of the cutting edge 4 is subject to a great load as mentioned above due to its proximity to the axis O when the tip is mounted on the tool body 7, it is necessary for the cutting edge 4 to secure sufficient cutting-edge strength at that portion. Otherwise, if the convexly-curved cutting edge portion 4A is provided at the one end 4a side of the cutting edge 4 to form a curve projecting in the direction of rotation T of the tool, breakage or chipping will occur at the time of biting into a work piece or during drilling work, making it necessary to replace the tip.
Description of another drawback to a conventional tip of this type will now be made.
A drilling tool, as mentioned above, is conventionally comprised of a substantially columnar tool body rotatable about an axis, a pair of chip discharge grooves formed in an outer periphery of the tool body, and tip-mounting seats formed at the distal end of the respective wall surfaces of the chip discharge grooves facing in the direction of rotation of the tool. A tip provided, for example, in the form of a substantially pentagonal plate is mounted on each tip-mounting seat, with one polygonal surface thereof facing as a rake face in the direction of rotation of the tool, and one side surface thereof facing as an end flank face the distal end of the tool body. A cutting edge is formed along the crossing ridge between the rake face and the end flank face.
In such a tip as mentioned above, another side surface that adjoins the end flank face and faces the outer periphery of the tool body is formed at the rake face side with a cylindrical surface that convexly curves from the rake face toward the other pentagonal surface (seat face) to provide a margin for use in drilling work. In other words, the margin, when the tip is mounted on the tool body, faces the outer periphery of the tool body and constitutes a part of a cylindrical surface centering around the axis of the tool body, and comes into sliding contact with the inner wall surface of a hole being drilled during drilling work to stabilize the cutting behavior, advancing the tool body, and improving the accuracy of the inner wall surface of the drilled hole.
With a drilling tool that uses such a tip as mentioned above, in order to realize improved stability of the cutting behavior, it is necessary to set the width and length of the margin of a tip to a certain size. A drawback to this is that the area of the margin that comes into sliding contact with the inner wall surface of a hole being drilled is enlarged, causing an increase in cutting force and a reduction in working efficiency. Because of the above, difficulty has been encountered in managing both to stabilize the cutting behavior to realize progressiveness of the tool body and to reduce the cutting force to improve the working efficiency.
Furthermore, with a drilling tool that uses such a tip as mentioned above, the margin is formed straight and is provided with a slight back taper, as viewed from a direction opposing the rake face of the tip, in other words, the margin is slightly inclined toward the inner periphery of the tool body as it proceeds toward the proximal end of the tool body.
As a result, if there is the slightest irregularity in the mounting accuracy of the tip resulting from variations in working accuracy of the tip-mounting seat, its impairment due to wear or the like, it possibly causes the back taper to be reversed; in other words, the margin possibly becomes inclined toward the outer periphery of the tool body as it proceeds toward the distal end thereof, so as to eliminate a relief from the margin. If this is the case, the cutting force encountered during drilling work becomes very large, resulting in instability of cutting behavior and occurrence of chatter or vibration, and deterioration of the inner wall surface accuracy of a drilled hole.
Furthermore, with a drilling tool that uses such a tip as mentioned above, the margin of the tip is formed to have a width, or a length of the margin in a direction perpendicular to the axis of the tool body, that is equal from the rear to the distal end side in the direction of the axis.
As a result, the margin does not have sufficient area at its distal end side near the cutting edge, and advance of the tool body may not be obtained due to this shortness of area at the distal end side of the margin, at the time of biting into a work when a greater cutting behavior stability is required, resulting in impairment of positional accuracy of a hole drilled. To cope with this, if the margin is formed with a larger width than usual, the rear end side portion of the margin now having an excessive area causes an increase in the cutting force due to its contact with the inner wall surface of the hole during drilling work.